Crack initiation in fatigue: experiments and three-dimensional dislocation simulations

In an attempt to better understand damage accumulation mechanisms in high cycle fatigue, a three-dimensional discrete dislocation simulation has been used both to simulate the dynamic evolution of the dislocation microstructure and the topography of the free surface where the plastic deformation is localised. The numerical tool is validated by comparing the dislocation structure obtained in double slip configuration to transmission electronic microscopy observation performed in 316L austenitic stainless steel. In the case of single slip loading conditions, the stress level obtained by the dislocation simulations are found to be consistent with results from the literature. After this validation stage, results of the dislocation simulations are analysed and a mechanism for the formation of intense slip bands is deduced. Finally, the computation of the relief of the free surface shows that extrusions and intrusions develop inside the bands, which demonstrates that plastic shear alone can give raise to crack initiation.